1. Technical Field
The present invention is generally directed to the detecting and measuring of a higher order degree of rotation of an articulated member undergoing multiple degrees of rotation. More particularly, the present invention is directed to the detecting and measuring of the pitch angle of a propeller blade. The first degree of rotation is defined as the blade rotation about the hub axis and is commonly known as propeller revolutions per minute (rpm). The second degree of rotation is defined as blade rotation about the longitudinal axis of the blade and is commonly known as pitch angle.
2. Prior Art
Rotating blade assemblies have long been used to convert rotating shaft power into useful work. One specialized and well known blade assembly is a propeller. A propeller normally comprises a hub rotated by a shaft. The hub usually has at least two propeller blades protruding from the hub such that the blades are orthogonal to the shaft. Each blade has an aerodynamic profile and is rotated through a fluid medium such that each blade generates a `lifting` force based on the Bernoulli principle. An important parameter in determining the amount of thrust generated by the propeller is the angle of attack of the blade. The angle of attack is critical in application of the Bernoulli principal because it determines the difference between useful work being performed or a stalled blade where no useful work is performed. It is well know in the art that without the proper angle of attack, increasing propeller revolutions per minute will not increase the amount of useful work output. Therefore the angle of attack is a critical parameter to be known in an operating propeller. The pitch angle of a propeller blade is that angle between the chord of the blade and the plane it rotates in about the hub. It is a very useful parameter in determining the angle of attack.
Early propellers were designed with each blade having a fixed pitch angle. Although the blade rotated about the hub at varying propeller RPM, its pitch angle remained constant. It soon became clear to the early inventors that due to the constantly changing environment a propeller operated in, a fixed pitch propeller was not a very efficient solution. Thus arose the impetus for inventors to discover ways to vary the pitch of each blade while the propeller was spinning. Once variable pitch propellers were discovered, it greatly changed the way propellers are used and controlled. Pitch angle is now varied dynamically for a variety of reasons. For example, when a power plant fails and shaft power is reduced significantly or eliminated, a fixed pitch propeller will create a large drag or negative thrust. However a variable pitch propeller can be `feathered` so that each blade is rotated until its chord is parallel to the shaft axis and therefore the blades generate only a small parasitic drag. Blade angle can also be varied to increase or reduce thrust without varying propeller rpm, i.e., a constant speed propeller. In some propeller assemblies pitch angle can be varied so that the blades generate reverse thrust, also known as the `beta` range.
An operator or control system is normally used to command the pitch angle of a propeller blade in an operating environment. Simultaneously, the operator or control system usually requires some feedback as to what the blade pitch angle is. Initially, feedback was a position indication of a control arm driving the mechanical linkage to alter pitch angle. This was actually an `open loop` system in that the operator adjusted a power setting and a propeller rpm setting without truly measuring a pitch angle. The control system would increase or decrease blade pitch to achieve the commanded propeller rpm setting. Thus the true blade pitch angle was unknown.
Variable pitch propeller systems typically incorporate a hub which encloses a chamber within its interior wherein a pitch angle change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch angle of the blades thereby altering the lift and drag characteristics of the propeller blades.
In most modern aircraft, the pitch change actuator is of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of the hydraulic fluid which drives the actuator. The adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by way of collateral apparatus, a change in fluid pressure whenever the monitored engine speed departs from the desired engine speed setting.
Such hydromechanical pitch change actuation systems are well known in the art. For example, commonly assigned U.S. Pat. No. 4,523,891 to Schwartz and Duchesneau discloses a conventional pitch actuation system wherein each propeller blade is operatively connected to a piston which is driven by the pressure of a fluid which is selectively directed in response to a departure from desired engine speed against the opposite faces of the piston thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston. The piston is reciprocally moveable within a cylinder disposed within the hub about a torque tube which extends from the fluid supply to the piston. As shown in U.S. Pat. No. 4,523,891, the pressure fluid is conveyed through a conduit within the torque tube from the fluid supply to a valve associated with the piston which, depending upon its position, selectively directs the fluid against either the front or the rear face of the piston thereby causing the piston to move linearly thereby rotating the blade or blades associated therewith to effectuate the change in pitch.
In the foregoing systems, however, there is no measurement of the actual blade pitch angle. The present invention provides a novel method and apparatus to solve this problem.